The invention relates to a device for feeding a fiber tuft mass (fiber batt) composed of, for example, cotton fibers, synthetic fibers or the like, to a fiber processing machine, such as a carding machine or a cleaner to prepare the fiber for spinning. The device has a fiber advancing mechanism formed of a feed roll and a cooperating feed table followed by at least one opening device such as an opening roll. The fiber advancing mechanism also serves as a batt thickness sensor. For this purpose there are provided a plurality of sensor elements which have one end attached to a rotatably or slidably supported, spring-biased holding member (summing beam) which summarizes the displacements of the individual sensor elements undergoing excursions as a function of the thickness variations of the fiber material which runs through the feed roll. The sensor elements constitute a guide assembly for the rotary or sliding motion of the spring-biased holding member. The sensor zone proper is formed at that end of each sensor element that is opposite to the end of attachment. The feed table is made of an extruded material such as aluminum or an aluminum alloy. Such a device is disclosed in the above-noted U.S. application No. 08/172,158.